The invention relates to a flat cathode ray tube comprising an electron gun to generate a low energy electron beam, first means to deflect the low energy beam in a first direction, means to accelerate the beam deflected in the first direction to a high energy beam, second means to deflect the high energy beam in a second direction and a trajectory control space which is formed by a screen and a repeller electrode provided at some distance from the screen.
The invention relates in particular to a low power flat cathode ray tube having a screen whose diagonal is typically 12.5 cm. By a flat cathode ray tube is meant one in which either the electron gun is disposed laterally of the screen and for convenience will be described as a single plane flat tube or the electron gun is disposed behind the screen so that the electron beam is bent through 180.degree. before it is deflected towards the screen.
Such a flat cathode ray tube is known from British Pat. No. 865,667 (PHB 30735) corresponding to U.S. Pat. No. 2,999,957. This patent describes embodiments of flat cathode ray tubes of various dimensions (for example, screen diagonal 50 cm), in which an electron beam is generated by an electron gun whose axis is inclined or parallel relative to the plane of the fluorescent screen. FIG. 14 of the above-mentioned patent corresponding to FIG. 27 of the '957 patent, shows an embodiment of a single plane cathode ray tube having an electron gun whose axis is at an acute angle to the edge of a trajectory control space formed between a phosphor screen and a repeller electrode. The electron beam leaving the gun is bent through said acute angle when being deflected in a first horizonal direction, is accelerated, and is deflected in a second, vertical direction as it enters the trajectory control space. The means for horizontally deflecting the electron beam comprises a slotted electrode and a parallel repeller electrode to define an auxiliary trajectory control space, the electrodes being inclined relative to the axis of the electron gun and the edge of the first mentioned trajectory control space. By maintaining a potential difference between these electrodes the beam from the electron gun passes into the auxiliary trajectory control space and follows a parabolic trajectory to pass through the slotted electrode along a path substantially normal to the edge of the first mentioned trajectory control space. Line deflection is achieved by holding the potential of the slotted electrode constant at 5 KV and varying the potential of the repeller electrode between 1.2 and 4.3 KV at line frequency, all the voltages being measured with respect to a 0 volt potential at the cathode, of the electron gun. After passing through the slotted electrode the beam is accelerated by another slotted electrode parallel to the first mentioned one and held at a potential of 15 KV. The accelerated electron beam passes through a triangular field free space, and then passes between a pair of frame deflecting electrodes which varies the angle of entry of the electron beam into the first mentioned trajectory control space in which the beam follows a parabolic trajectory to then impinge on the phosphor screen. Typically the phosphor screen is at a constant potential of 15 KV whilst that of its associated repeller electrode is at a lower voltage.
The above described cathode ray tube of British Pat. No. 865,667 is essentially a high voltage tube and can be used in situations where power consumption is not of vital importance. A high energy beam is produced by the electron gun and, in order to bend the beam through said acute angle whilst at the same time swing the voltage between 1.2 and 4.3 KV, a reliable high voltage driver is required which is expensive. Since the known cathode ray tube is of a relatively large size it is possible to accelerate the electron beam over a relatively large space, say 10 KV over 7.5 to 10 cms, without adversely affecting the spot size.